target/mips: Remove XBurst Media eXtension Unit dead code

All these unimplemented MXU opcodes end up calling
gen_reserved_instruction() which is the default switch
case in decode_opc_mxu().

The translate.c file is already big enough and hard to maintain,
remove 1300 lines of unnecessary code and /* TODO */ comments.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210226093111.3865906-5-f4bug@amsat.org>